Existing power generation systems have various inherent limitations, for example, thermal power generation emits a large amount of greenhouse gases and environmental pollution by using fossil fuels, and nuclear power plants have stability issues and the need for hazardous waste processing. In response thereto, research efforts to develop environmentally friendlier, higher efficiency energy sources, and a power supply system using the same, have increased significantly.
In particular, power storage technology has been the focus of research and development activities for broadening the usability of renewable energy sources with respect to their significant susceptibility to external conditions and for enhancing efficiency of power utilization, wherein secondary batteries receive more intensive interest and research and development efforts thereof are being actively undertaken.
A redox flow battery is an oxidation/reduction cell capable of converting chemical energy of an active substance directly into electrical energy, and it represents an energy storage system adapted to store new and renewable energy with substantial output variations according to environmental conditions such as sunlight and wind, and to convert the same into high-quality power.
Specifically, the redox flow battery has electrolytes containing an active material that causes an oxidation/reduction reaction, and circulating between opposite electrodes and a storage tank, to perform charging and discharging.
Such a redox flow battery typically includes tanks containing active materials in different oxidized states, a pump for circulating the active materials during charge/discharge, and unit cells partitioned by a separation membrane, wherein the unit cell includes electrodes, an electrolyte, a current collector, and a separation membrane.
The electrolyte includes an active material that undergoes the oxidation/reduction process for enabling the charge/discharge operation, serving as an important factor to determine the battery capacity.
For example, a vanadium flow battery has an electrolytic solution composed of four ions with different oxidation numbers. During the process of charging and discharging, a hydrogen ion accompanied by water or vanadium ions may permeate the separation membrane, which is an ion crossover phenomenon, to generate a self-discharge. When this phenomenon occurs, different moving speeds of respective ions cause a density difference and a volume difference between opposite electrodes. Then, the ill-balanced opposite electrolytic solutions lead to a reduced absolute amount of redox coupling, and degradation of battery performance and a charge retention rate.